From what I understand the idea was that innocent people would sink, and witches would float (when tied up and thrown into water). The reason being that since witches had "rejected" the waters of baptism, water would "reject" them. There's probably more to the story though.

I believe it goes; "Innocents sink, but witches 'float'", Obviously nobody floated, and when they drowned, they were found innocent. Posthumously, of course. Thereby guaranteeing that merely being accused was a death sentence, which I think was the point.